A projection photolithography machine is a device for projecting a pattern on a photomask onto the surface of a wafer by using a projection objective. The performance of an exposure process carried out by a photolithography machine will be seriously impaired if some areas of the wafer surface in an exposure field are not within an effective depth of focus (DoF) due to a deviation or tilt of the wafer from a focal plane of the objective. For this reason, a focusing and leveling system is adopted to precisely control the position of the wafer within the exposure field. Existing focusing and leveling systems can achieve this by acquiring values of height and tilt of the wafer surface within the exposure field, determining whether correct focusing and leveling has been attained based on the values and making appropriate adjustments if necessary, so as to precisely control the position of the wafer based on the determination.
As shown in FIG. 1, in a conventional scanning mirror-based focusing and leveling system, a measuring beam emanated from an illumination unit 101 passes through a projection slit 102 and is reflected by a first planar mirror 103 onto the surface of a wafer 104, forming thereon a light spot. The reflected beam from the surface of the wafer 104 is further reflected by a second planar mirror 105 and is then incident on a scanning mirror 106 which periodically vibrates in a simple harmonic motion and thus modulates the optical signal to improve its signal-to-noise ratio (SNR). After leaving the scanning mirror 106, the beam propagates through a detection slit 107 and strikes a photodetector 108 which then outputs a voltage signal produced based on the received optical intensity. As a result of the modulation effectuated by the scanning mirror 106, the voltage signal output from the photodetector 108 is a periodic dynamic voltage signal. Finally, this dynamic voltage signal is analyzed to detect an amount of defocus of the surface of the wafer 104. The scanning mirror 106 serves as a reference for the focusing and leveling system and is a very critical motion component thereof because its stability in terms of scanning amplitude and position directly determines the system's overall performance. Under the effects of the stability of control and actuation signals, material fatigue of a pivoting axle of the mirror 106 over time, and changes in the ambient temperature and pressure, the vibration amplitude and position of the scanning mirror 106 may vary and hence adversely affect the performance of the focusing and leveling system.
Currently, a common solution for the problem arising from variations in the vibration amplitude and position of the scanning mirror is to irregularly assess and calibrate the focusing and leveling system. In other words, measurement errors and other adverse outcomes arising from variations in the vibration amplitude and position of the scanning mirror are eliminated by means of calibrating the focusing and leveling system from time to time. However, shifts of the object under test and status changes of the scanning mirror are generally coupled together, so that this method is disadvantageous in being incapable of accurately distinguishing whether there is a status change in the scanning mirror. Another conventional solution allows calibration and adjustment of only the vibration amplitude of the scanning mirror. It, however, requires a wafer stage system of the photolithography machine to be involved and requires the machine itself to stop performing the ongoing task. There is still another conventional solution in which a separate monitoring device is employed to monitor and calibrate the vibration amplitude of the scanning mirror. This approach, however, can only monitor the vibration amplitude of the scanning mirror without doing anything to positional shifts of the scanning mirror itself, which may also deteriorate the measurement accuracy of the focusing and leveling system. Therefore, there has not been proposed, in the prior art, a solution capable of measuring and controlling, in real time, variations both in the vibration amplitude and position of the scanning mirror.